The ability to manipulate materials at nanometer-length scales and control the dimensions of nanostructures is a prerequisite of not only studing novel properties of materials at different length scales but also of realizing useful miniaturized devices. Nanopatterning of materials is one approach that enables these dual goals. Functional ceramics such as ferroelectric (e.g., PbZrxTi1-xO3 known as PZT), ferromagnetic (e.g., CoFe2O4 known as CFO), and optoelectronic materials (e.g., ZnO) are very important technological materials for applications such as actuators,’ chemical sensors, high density data storage, and polychromic displays. Some of these properties show remarkable size dependency as well as interesting synergistic coupling when materials with different functionalities are positioned in close proximity. The ability to create nanoscale architecture for functional ceramics is desirable.
Although several nanopatterning schemes have been developed during the last two decades, techniques that can pattern ceramics under 100-nm resolution are very limited. This is due in part to the refractory nature of ceramics and the difficulty in etching such materials. Dip-pen nanolithography (DPN) with sol-gel inks has been employed to generate structures under 200 nm. Another high-resolution nanopatterning technique for ceramics is the direct-write using an electron beam. Although this technique was shown to generate very fine structures (<10 nm), it is limited to resists that are sensitive to e-beams and the process typically requires a high electron dose for development of e-beam-sensitive inorganic resists. Other patterning techniques based on molding and replication processes generate ceramic structures over fairly large areas, and direct-write fabrication techniques such as robotic deposition of polyelectrolytes could generate 3D ceramic architectures. However, these techniques do not attain true nanometer-scale patterning resolution.
The past decade has witnessed the emergence of diverse techniques for patterning a wide variety of molecular and “soft” nanostructures within a two-dimensional (2D) regime. However, as the nanostructure paradigm shifts from monolithic single-phase materials and planar geometry to complex compounds and stacked 3D architectures, there is a continuing need for advancing the nonplanar stacking of solid-state structures, especially for multifunctional materials, in a patterned geometry. Heterostructures composed of multifunctional oxides (e.g., multilayer structures) exhibit a fascinating breadth of properties for applications such as microelectromechanics, optoelectronics, microwave devices, and data storage, among many others, based on the interaction between different phases when stimulated by external fields. Recently, vertically aligned heterostructures (such as nanopillars of one phase embedded in the matrix of another) have attracted considerable attention as these nanostructures significantly enhance such interactions. While these vertical heterostructures are fabricated using a phase-separation-based self-assembly approach, a patterning technique that affords controlled dimensions and the separation of such minute structures is highly desirable.
There are considerable challenges in fabricating nonplanar ceramic heterostructure nanopatterns using current “top-down” patterning techniques, which are better suited for the fabrication of single-component 2D structures. The key bottleneck is the stringent requirement on feature realignment between the multiple patterning steps, which are required in order to form nonplanar patterns composed of more than two materials. Such a constraint is particularly critical for nanometer-sized structures as it allows alignment precision at a relatively small scale. Methods such as polymer phase separation, templated growth, interference lithography, and nanotransfer printing have been developed for fabricating nonplanar nanostructures without the need for high-precision feature alignment. However they are demonstrated mainly for metals and polymers, and are not well suited for ceramics. Direct deposition methods such as robotic deposition enable the fabrication of intricate ceramic structures, yet there are restrictions on the deposition environment and the smallest feature size attainable.